Summary

Murine leukemia cells (L5178Y) sensitive to l-asparaginase become resistant to the killing effect of methotrexate (MTX) following l-asparaginase treatment, with a decrease in the ability of MTX to inhibit the incorporation of deoxyuridine in DNA. Injections i.p. of l-asparaginase, 250 units/kg, into BALB/c × DBA/2 F1 mice bearing the L5178Y tumor produced a decrease in in vitro uptake of MTX within a few hr. Uptake in treated mice was 75% of that of controls 24 hr after treatment. However, cells recovered their ability to transport MTX 96 hr after l-asparaginase. Decrease in the activity of dihydrofolate reductase, the MTX target enzyme, in the treated cells was insignificant. Whether this effect on the carrier-mediated transport of the nonmetabolizable MTX is specific was determined by studying the effect of pretreatment with l-asparaginase on cycloleucine, which is a nonmetabolizable amino acid; and also of cytosine arabinoside, 6-mercaptopurine, and 5-fluorouracil (none of which is actively transported and all of which are later converted to nucleotides by cellular enzymes). All four exhibited a decrease in cellular levels. The demonstrated inhibition of cellular levels of the antineoplastic agents when mice were treated with l-asparaginase is dose dependent and can be blocked by administration of l-asparagine. Such an effect of l-asparaginase is nonspecific and may be due mainly to its primary effect on protein synthesis.

Footnotes

↵1 This publication is Paper 1 in the series, “l-Asparaginase in Combination Therapy of Murine Leukemia L5178Y.” This work was supported by Grants CA-13712 and CA-11198 from the USPHS.

↵2 To whom requests for reprints should be sent, at Department of Biochemistry and the Cancer Center, University of Rochester School of Medicine, Rochester, N. Y. 14642.